As classic filtering methods low-pass filters having constant coefficients, are known for obtaining the useful signal. However, these low-pass filters do not give any useful filtering in case of very low frequency noise and short measuring time.
In the book by A. Gelb "Applied Optimal Estimation", pp. 124 to 126, an optimal filter is described, in which the difference of the measuring signal and an estimate of the measuring signal is integrated with a factor varying according to a certain function of time, the integral supplying said estimate of the measuring signal. Such type of filtering supplies improved results as compared to the conventional filtering methods.
DE-B-24 41 926 describes a method for determination of signals influenced by disturbances, in which the signals are integrated and the integration value obtained during an integration time is divided by the integration time and indicated or further processed in some other way. The integration time is made dependent on a stabilization of the indicated value.
DE-A-30 40 753 describes a method for the determination of a direct voltage superimposed by a disturbing alternating voltage, in which the voltage to be determined is integrated. Therein, the integration is effected between defined phases of the disturbing alternating voltage.